Every additional law added on the books results in additional overhead/burden to the entire legal system, so if a law doesn't have much (if any) of a positive effect, it shouldn't be added. Furthermore, you're basically condoning sending someone to prison for repeat violations of any law, so by endorsing this law you are in effect saying "I am okay with someone going to prison for repeatedly selling violent video games to minors." I'm not OK with that, so I cannot support this law, it's that simple. A few other notes:
1) Major retailers will not sell minors violent games anyway. In fact, I've never come across a retailer that will. Even now I get asked for my ID when I buy a game. If it's corporate policy to not do this already, in general, why bother enshrining it into law?
2) Violent video games have not been proven to be detrimental. Studies on the subject trend in favor of them being pretty harmless from what I've seen (not to say that there aren't a few that go the other way, that's why I said trend). Why are we restricting access to something that isn't proven harmful?
3) There are plenty of other safeguards put in place, so this additional measure will only be strengthening security by a bit.
4) Continuing from 3, there are still plenty of methods for kids to get access to violent games even if they don't buy them at retailers. This law will have to be expanded to affect the downloadable market, eventually the internet, and even then there are still free violent flash games on the internet. Are we gonna start regulating the internet to shut off the flow?
5) Every law passed restricting video games makes it a shorter and shorter jump to the next point of legislation that video game opponents want enacted. I don't want to set any precedence to assist in more potential media censorship.
In short, I think that this law is unnecessary, pointless, and ineffective.